Hydraulic fluid applications are well known in the automotive industry. Two well known hydraulic fluid applications are automotive transmission systems and automotive braking systems. The pressure of the hydraulic fluid in such systems is commonly controlled and varied through the use of actuators. Hydraulic fluid pressure is known to be a vital element in the proper performance of such systems.
In an automotive automatic transmission system, electronic transmission controls are commonly used to control the pressure of the hydraulic fluid line. The pressure of the hydraulic fluid line can be used to control various elements of the automatic transmission system including the engagement of individual planetary gears. By engaging various combinations of planetary gears, an automatic transmission system accomplishes the same task as the shifting of gears in a manual transmission. Therefore, it is common within the art to refer to changes in the status of the planetary gear system as shifting gears or shift points even though the mechanisms are not commonly the same as those found in manual transmissions. It is known that the pressure of the hydraulic fluid line in automatic transmission systems can be used to control the shift points.
Known electronic automatic transmission systems often control the shift points based upon various factors including engine RPM, engine torque and vehicle speed. The electronic control unit (ECU) in these systems use the data from these factors to determine an optimal hydraulic fluid line pressure. Once the optimal hydraulic fluid line pressure is determined by the ECU, look-up tables within the ECU are accessed to determine the correct level of current to send to the actuator controlling the hydraulic fluid line pressure. These systems continue to monitor the data factors as well as the shift timing to determine if the correct hydraulic line pressure was achieved. If the data relayed to the ECU indicated that the correct hydraulic line pressure was not achieved, a new value is looked up in the tables and an "adapt" is recorded such that future adjustments are expedited. This system must continually zero in on the actual line pressure value, as it does not provide any way of monitoring the line pressure directly. This process of continually making adjustments is inefficient, may have a negative effect on the shift-feel quality of the transmission system and can have a negative effect on the fuel economy of the automobile.
In addition, it is known that in some existing systems, the "adapts" may not exceed 10 psi and therefore require the transmission to be occasionally recalibrated. Such a system is highly undesirable and can lead to customer dissatisfaction. It would therefore be desirable to have a transmission control system that would quickly, efficiently and consistently set the hydraulic fluid line to the correct pressure and thereby provide improved shift-feel quality and improved fuel economy and would minimize service requirements.
It is known that a similar system of hydraulic fluid line pressure control is utilized in some modern braking systems. These braking system use a traction control module (TCM) to adjust the brake fluid line pressure based upon various sensors that provide data relating to driving conditions. These braking systems do not monitor the actual pressure within the brake fluid line but rather provide approximations from look-up tables and continual adjustments to zero in on the actual line pressure value. Therefore, these systems share the same inefficiencies found in the above described automotive transmission systems.
Therefore, there is a need for an actuator that can quickly, accurately, and consistently set the line pressure in a hydraulic fluid line without the inefficiencies and maintenance requirements of known designs.